There is increasing enthusiasm for clinical trials aimed at early intervention in the pre-clinical IDDM patient in order to halt or slow the decline in beta cell function that occurs with autoimmune attack. Such trials generate a need for two kinds of phenotypic markers. First, there is a need for markers that allow prediction of who will develop disease in the general population where the incidence of IDDM is low. Reliable, specific markers of this nature would allow the size of intervention trials necessary to demonstrate the efficacy of proposed therapies to be reduced. Second, in clinical intervention trials of pre- diabetics as currently contemplated, therapeutic efficacy is mainly determined by whether or not the patient becomes diabetic over 3-5 years, or more. There is an additional critical need for surrogate markers. which will reflect therapeutic efficacy sooner than that. In this project, we hypothesize that cellular immunity to glutamic acid decarboxylase (GMD) may be an important predictor of progression to IDDM. A limiting dilution assay will be used to measure the frequencies of T cells reactive to GMD in serial samples of peripheral blood lymphocytes from prediabetics (defined based on multiple marker positively) and HLA matched controls. GMD T cell precursors will tested for their frequency, peptide specificity, and cytokine (IL-2 vs. IL-4) secretion longitudinally. T cells reactive to GMD epitopes will be cloned by several methods including a novel molecular approach. Finally, TCR usage in these clones will be determined and an in situ PCR/FACS assay will be used to measure the frequencies of GMD reactive TCR bearing T cells in the serial PBL samples from prediabetics and controls. The goal of these studies to develop and validate assays for cellular immunity to GMD as new predictive tools for IDDM progression. This is Project #2 of an interdisciplinary diabetes research program. As such it benefits from, and contributes to, a number of collaborative and interactive studies of early immunologic events in IDDM pathogenesis. For example, this project will rely heavily on the clinical core (A) to provide well characterized patient material for study, and will depend on the other projects for the identification of GMD peptides that bind to DQ3.2 (project #1), and peptides preferentially processed and presented by different types of antigen presenting cells (projects #3 and #4). In turn, this project will provide recombinant human GMD and GMD reactive T cells to the other projects and serve as a testing ground for the predictive and therapeutic utility of data derived from those projects.